I can understand your confusion about whether or not he actually has congestive heart failure with secondary pulmonary edema but even if he does have these problems, they shouldn't cause issues with his back legs. Weakness, perhaps, but I wouldn't expect him to be dragging a back leg or have difficulty walking. Nor would I expect these conditions to be painful.
Based on your description of his behavior, it sounds as if he may have issues with his spine (compression of the cord by a disc or mass) or possibly he's exacerbated a previous arthritic condition since I'm fairly certain he has arthritis somewhere in his body. There are other conditions which might also explain his behavior such as:
tick diseases or
Discospondylitis (infection along the vertebrae) or
Fibrocartilaginous embolic myelopathy (or FCE which is a condition involving necrosis or cell death of a region of the spinal cord secondary to infarction (obstruction) of the blood supply.
But I'd want to rule out spinal issues and/or arthritis first. This isn't always the easiest thing to do, even with diagnostic imaging. -X-rays may be suggestive of the first but not diagnostic....although they're usually pretty accurate at detecting arthritis. Often an MRI or CAT scan are required to definitively determine if there's a spinal cord problem.
Spondylosis might cause some issues but not to the extent that you're seeing, at least not in my experience.
I must confess to being somewhat disappointed with your vet that they didn't examine him thoroughly or take xrays since he was there. Even though he may have shown improvement at the time, I would have wanted to know what was going on. But, be that as it may now, I do have a few suggestions if you can't get him back to the vet's office and since he seems to be so uncomfortable.
1. Since I don't see where he's taking any steroids or nsaid medication and he's not vomiting (I assume), then otc Aspirin can be given which may be of help for it's anti-inflammatory and anti-pain properties. The dose would be 10 mgs per pound of body weight twice a day with food to avoid stomach upset.
I honestly don't know if it will be strong enough to provide relief for him but it may help to a certain degree. One hesitation I have about it's use would be if you manage to get him back to the vet's office and they want to prescribe something different for him; Aspirin may interfere with this medication but several doses should be fine.
I don't like to use nsaid medication long term for my cardiac patients if can avoid it but sometimes we have to balance the pros and cons of doing so when there's a patient with multiple issues...such as in this case.
2. If you have access to specific pain medication such as Tramadol or Gabapentin, I can provide the dosage amount of I know his weight. It would be illegal for me to advise the use of a drug which had been prescribed for another patient (human or otherwise) but I can provide the current dosage for a canine patient.
3. I wouldn't necessarily discontinue any of the drugs you mentioned since they aren't likely to be causing his current back end issues. I'd worry if you discontinued them abruptly without discussing this with your vet first.
4. It might be helpful to take his temperature since I wouldn't expect it to be elevated with a spinal or arthritis issue. Normal is between 100 and 102.5 degrees Fahrenheit.
I do have a long list of supplements that I typically recommend for my patients (in addition to the joint supplement he's taking) which I'll be happy to provide; however, they aren't going to help him now since they take weeks to build up in the system (which you probably already know).
It certainly sounds as if he's in a bad way, unfortunately, and I'm not certain if what I've suggested above will be enough to improve his situation but if you can't get him back in the car for a recheck vet visit, then hopefully the Aspirin alone (if that's all you have) will help him.